scanner: Generate macros for getting the 'since' version of an event

This could be useful for compositors who need to be able to not send
events if the client bound a version lower than the newest provided.

Event version numbers are exposed as
[INTERFACE_NAME]_[EVENT_NAME]_SINCE_VERSION for example wl_output.scale
will have the version macro WL_OUTPUT_SCALE_SINCE_VERSION.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>